The invention relates to a device for digitally detecting one or more of a plurality of fixed frequencies in an analog signal according to a correlation method, the fixed frequencies occurring in a first group of lower frequency tones and in a second group of higher frequency tones, and wherein the third harmonic of one or more frequencies of the first group is equal or almost equal to a frequency of the second group.
Similar devices are generally known and are used particularly for detecting MFC (mult-frequency code)-signalling between telephone exchanges.
The object of the invention is to provide a device of the sort described, but with a greater reliability than the known devices, and offering the possibility of making the detection in a relatively short time, use being made of the method already known from the Proudfoot U.S. Pat. No. 3,882,283 issued May 6, 1975.
A problem attendant on devices of the sort is that by making use of the correlation principle known from said Proudfoot patent, the detection of odd harmonics takes place because the reference signals are stored as squarewave voltages. When one of the frequencies of the second group is equal or almost equal to an odd harmonic of one of the frequencies of the first group, the frequency of the second group can be detected as the frequency of the first group. In order to prevent this, the correlation time is extended according to a known technique, due to which a better discrimination of the correlation products for each of the frequencies can be achieved. This extension of the correlation time is poorly compatible with the desirability of a time-division use of the detection device.